Such RF systems, furnished with a rotary variable capacitor, have been known for a long time, for example from patent GB-655271. It is also known that the rotor of the capacitor is prone to heating which is in part due to the eddy currents which appear in the rotor subsequent to the rotary motion of the latter in the magnetic field of the synchrocyclotron, that is to say in the magnetic field which makes it possible to maintain the particles in their trajectory within the synchrocyclotron. Other causes of heating are the RF currents which traverse the rotor. Now, this heating causes deformations in the geometry of the rotor, and this may disturb its proper operation. It can also lead to premature aging of the materials of which the capacitor is composed and/or which are in contact with it.
In the known RF systems, the rotor is generally cooled by water, as described for example by K. A. Bajcher et al, (“improvements in the operational reliability of the 680 mev synchro-cyclotron as a result of the modernization of its rf system”; joint institute for nuclear research, Dubna report 9-6218). The rotor can also be cooled by air and water, as described in “design of the radio-frequency system for the 184-inch cyclotron” by K. R. MacKenzie et al. However, this system requires the manufacture of a complex labyrinthine network of flexible pipes and the addition of air blowers and a system for evacuating this air.
Such cooling systems are complex, expensive, and often rather unreliable. They are also sometimes inadequate and then require additional thermal protection measures for sensitive elements. In this regard, Bachjer et al. describe for example that they furnish certain parts of the capacitor with magnetic screens so as to limit the eddy currents in these capacitor parts. These known cooling means are moreover increasingly difficult and expensive to implement as RF powers increase and/or the rotation speed of the rotor increases, this being the case in the synchrocyclotrons which are undergoing development and which are aimed at increasing the number of packets of particles that they can produce per unit time.